The present invention relates to inkjet printing. More particularly, the present invention relates to a method and apparatus for evacuating an ink chamber for an inkjet printhead.
An inkjet printer for inkjet printing includes a pen in which small droplets of ink are formed and ejected towards a print medium. Such pens include a printhead having an orifice member or plate that has a plurality of small orifices through which ink droplets are ejected. Adjacent to the orifices are ink chambers, where ink resides prior to ejection through the orifice. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be contained in a reservoir portion of the pen or in a separate ink container spaced from the printhead in the case of "off-axis" ink supplies.
Ejection of an ink droplet through an orifice may be accomplished by quickly heating a volume of ink within the adjacent ink chamber. This thermal process causes ink within the chamber to super heat and form a vapor bubble. Formation of the vapor bubble is known as "nucleation". The rapid expansion of the bubble forces ink through the orifice. This process is sometimes referred to as "firing". The ink in the chamber is typically heated using a resistive heating element which is positioned within the chamber.
Once ink is ejected, the ink chamber is refilled with ink from an ink channel which is in fluid communication with the ink chamber. The ink channel is typically sized to refill the ink chamber quickly to maximize print speed. Ink channel damping is sometimes provided to dampen or control inertia of the moving ink flowing into and out of the chamber. By damping the ink flow between the ink channel and the ink chamber underfilling and overfilling of the ink chamber resulting in meniscus recoiling and bulging, respectively, can be avoided or minimized.
As the vapor bubble expands within the ink chamber the expanding vapor bubble can extend into the ink channel. Expansion of the vapor bubble into the ink chamber is known as "blowback". Blowback tends to result in forcing ink in the ink channel away from the ink chamber. The volume of ink which the bubble displaces is accounted for by both the ink ejected from the nozzle and ink which is forced down the ink channel away from the ink chamber. Therefore, blowback increases the amount of energy necessary for ejecting droplets of a given size from the ink chamber. The energy required to eject a drop of a given size is referred to as "Turn-On Energy" (TOE). Printheads having high turn-on energies tend to be less efficient and therefore, have more heat to dissipate than lower turn-on energy printheads. Assuming a given ability to dissipate heat then printheads that have a higher thermal efficiency are capable of a higher printing speed or printing frequency than printheads which have a lower thermal efficiency.
The turn-on energy is a sufficient amount of energy to form a vapor bubble having sufficient size to eject a predetermined amount of ink from the printhead orifice. The vapor bubble then collapses back into the ink chamber. Components within the printhead in the vicinity of the vapor bubble collapse are susceptible to cavitation stresses as the vapor bubble collapses between firing intervals. Particularly susceptible to damage from cavitation is the heating element or resistor. A thin protective passivation layer is typically applied over the resistor to protect the resistor from stresses resulting from cavitation. A problem with the use of a passivation layer for preventing or limiting cavitation damage is that this passivation layer tends to increase the turn-on energy required for ejecting droplets of a given size.
There is an ever present need for printheads which have a high thermal efficiency and are capable of printing at high print frequencies. These printheads should be reliable and capable of extended printing without failure. In addition, these printheads should be relatively easily manufactured so that the overall cost of the printhead is relatively low.
Finally, these printheads should be capable of forming high quality images on print media. These printheads should be capable of forming droplets having the same or nearly the same drop volume over a wide variety of inks used in the printhead. For example, the printhead should be capable of providing a selected droplets volume regardless of the ink surface tension or the ink viscosity. This allows the same printhead to be used for a variety of different printing applications. In addition, the droplets formed by the printhead should not have tails which tend to result in splattering, puddling and generally poor image quality. Furthermore, these printheads should be capable of minimal trajectory errors which tend to result when the ink droplets are not well defined during ejection.